classes/populate_sdk_ext: add symlinks and unfsd to support Eclipse plugin

The Yocto Project Eclipse plugin requires that runqemu and unfsd are
accessible within the SDK, and indeed the standard SDK has these. This
turns out to be fairly easy to do - we just need to add unfsd and symlink
it, runqemu and a few other scripts into the SDK's bin directory.
Fixes [YOCTO #10214].
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>